1. Field of the Invention
The present invention relates to a DC-DC converter capable of operating in a pulse-skipping mode, and more particularly, to a DC-DC converter capable of automatically adjusting a width of a pulse-skipping-mode control signal according to an input voltage and an output voltage.
2. Description of the Prior Art
Please refer to FIG. 1, which depicts a schematic diagram of a prior art DC-DC converter 100. The DC-DC converter 100 provides power for a load circuit 112, and includes a driving circuit 102, a converter circuit 103, and a feedback circuit 110. The converter circuit 103 includes transistors 104 and 106, and an inductor 108. The driving circuit 102 drives the transistors 104 and 106 according to a feedback signal from the feedback circuit 110, so as to adjust an output voltage VOUT of the inductor 108. Firstly, the driving circuit 102 turns on the transistor 104 and turns off the transistor 106 so that the source current of the transistor 104 flows into the inductor 108 to store energy. Subsequently, the driving circuit 102 turns off the transistor 104 and turns on the transistor 106 so that the inductor 108 does not receive energy. Inverse current corresponding to the source current is generated and flows through the transistor 106 and down to a system ground. Therefore, the driving circuit 102 can adjust the output voltage VOUT of the inductor 108 by switching on and off of the transistors 104 and 106.
Therefore, electronic devices can be operated with power provided by the DC-DC converter 100. However, as energy-saving aspects become significant, how to reduce power consumption when output load is light has become an objective of industrials. The prior art has developed some methods, such as pulse-skipping-mode (PSM), burst-mode, and off-time-modulation techniques, for reducing switching and rated losses of a DC-DC converter under light or no load conditions.
The PSM technique, provided by SGS-Thompson and National Semiconductor, is achieved by skipping switching pulses according to a load condition, so as to reduce power loss. Please refer to FIG. 2, which depicts a schematic diagram of the PSM technique. The main concept of the PSM technique is to skip a part of switching pulses when a system operates with or without a light load, so as to decrease switching frequencies of transistors under light load conditions.
Therefore, the PSM technique can reduce power consumption of the DC-DC converter 100 under light load conditions by decreasing the switching frequencies of the transistors. The prior art has disclosed related circuits for implementing the PSM technique. However, the prior art mostly adopts a control method, constant inductor peak current, and thereby a peak current of the inductor 108 is restricted in a specific range. Under this circumstance, in order to avoid the situation that the current of the inductor 108 in the pulse-skipping mode is larger than that in a normal mode, or a continuous conduction mode (CCM), inductance of the inductor 108 is under restriction, resulting in inflexibility on selecting the inductor 108.
For example, U.S. Pat. No. 5,745,352, “DC-to-DC Converter Functioning in a Pulse-skipping Mode with Low Power Consumption and PWM Inhibit”, discloses a DC-to-DC converter capable of operating in a pulse-skipping mode, which outputs constant peak current of an inductor. The DC-to-DC converter of U.S. Pat. No. 5,745,352 detects voltage of an external resistor for obtaining current information and compares the obtained current information with a constant voltage, so as to adjust a peak current. When an output voltage is lower than a threshold value, transistors are switched and current flows into the inductor. Then, current of the inductor is accumulated and outputted to a load at last. A shortcoming of the DC-to-DC converter is that the current of the inductor in PSM is possibly larger than that in CCM. Thus, inductance of the inductor is under restriction.
Furthermore, U.S. Pat. No. 6,661,679, “PWM Controller Having Adaptive Off-time Modulation for Power Saving”, discloses a PWM controller capable of saving power via an off-time modulation. The PWM controller firstly detects a feedback voltage and an input voltage of a load, transforms the feedback and input voltages into currents, and makes use of currents carrying information of the feedback and input voltages for modulating off-time. Thus, off-time is a function of two variables, the feedback and input voltages, while on-time is determined by a period of a constant peak current, similar to the DC-to-DC converter of U.S. Pat. No. 5,745,352. U.S. Pat No. 6,661,679 only describes the relation that off-time is proportional to the feedback and input voltages, but does not illustrate the relation between on-time and off-time in CCM.